Page 215 - Vitamin D and Cancer

202 F.S.G. Cheung and J.K.V. Reichardt

compared to those who were born in Australia, whereas migration after the age of
10 had a quarter of the rate of native-born Australians [71]. Risk for all three types
of skin cancer also showed a positive correlation with ambient solar radiation and
increasing average annual hours of bright sunlight though the extent of this correla-
tion seems to vary depending on the type of skin cancer [4]. The frequencies of all
three cancers were generally the greatest on high sun-exposed body sites such as
the face, ears and neck and low on the rarely exposed sites [5, 59]. Interestingy, the
densities for melanoma and BCC are higher on the more intermittently exposed
shoulders and back while SCC has a lower density on these sites and is higher on
the back of the hands. This association is consistent with results of the study on the
relationship of personal sun exposure with skin cancer risk. SCC is strongly related
to total sun exposure and occupational sun exposure (continuous pattern of expo-
sure), while melanoma and to a lesser extent, BCC, show significant associations
with non-occupational/recreational (intermittent) exposure and sunburn (intense
intermittent exposure) [46]. Thus, with the evidence that SCC, BCC and melanoma
is caused by sun exposure, it is of no surprise that a latitude gradient of skin cancer
exists, with increasing incidence and mortality rates corresponding with increasing
proximity to the equator [25, 91]. The magnitude of the latitude gradient was
approximately 65% and 50% greater in incidence and mortality of melanoma
respectively, for body areas most intermittently exposed compared with those with
a least intermittent pattern of exposure [24].
Although there is a vast amount of persuasive evidence that support the classical
belief that sun exposure causes skin cancer, a recent study by [17] provided a new
school of thought on the relationship of sun exposure and skin cancer development.
A number of previous studies have shown that that the incidence of cutaneous
melanoma varies by season with a peak in summer [16, 20, 126, 136, 137]. It has
been hypothesized that if the higher incidence in summer is due to increased aware-
ness and detection of lesions on exposed skin, thinner lesions will be seen; whereas
a late stage promotion effect from the summer sun will yield thick lesions with
worse prognosis independent of Breslow thickness. Although increased thinner and
less aggressive lesions were indeed found in younger women during summer which
seems to correlate increased incidence with awareness, there was still a significant
increase of 18% in incidence for the constantly exposed head and neck. Thus, the
data do not exclude the possibility of greater awareness in summer or a late-stage
promotional effect of sun exposure (consistent with the classical belief). Interestingly,
the same study also found a significant 20% of reduced fatality for melanomas
diagnosed in summer to those diagnosed in winter. These rates were independent
of seasonal thickness variation, age, sex, anatomical site and histologic type of the
melanoma [17]. Therefore, these results are suggestive of a more complex pathway
in the development or progression of melanoma that is not restricted to the classical
effects of direct sun exposure [17].
Consistent with the results obtained by Boniol et al. were the results found by
[9] who conducted a study to investigate the effect of sun exposure on melanoma
fatality. This study showed that solar elastosis, sunburns and intermitted sun expo-
sure were inversely associated with melanoma fatality. This finding was also